VIOLENT aggression in humans may be because of a genetic defect, a recent Dutch study suggests. Han G Brunner and his colleagues at the University Hospital in Nijmegen report that a change in the gene coding for an enzyme called monoamine oxidase A (MAOA) may be responsible for unprovoked, aggressive outbursts displayed by certain males in a Dutch family that the researchers have been studying for the past ten years (Science, Vol 260, No 5115).

To many geneticists, the link between an MAOA gene defect and abnormal behaviour sounds plausible because the enzyme encoded by the gene helps break down several chemicals that in excessive amounts can cause a person to turn violently aggressive.

Previous studies linking the gene and behaviour were not conclusive. For one thing, humans have two genes for monoamine oxidase enzymes -- the other being MAOB -- and the two are located next to each other on the X chromosome.

But the Nijmegen group has biochemical evidence indicting the MAOA gene defect. They found that affected men secreted abnormal amounts of chemicals that the two enzymes work on. Presumably, MAOA is the enzyme at fault because the men's MAOB variant appears normal. But geneticists caution it would be premature to apply these results to the population at large.